In our copending application Ser. No. 347,341 filed Apr. 2, 1973, now U.S. Pat. No. 3,834,503, we have disclosed a dual hydraulic coupling to be used, for example, in an automotive transmission in which an engine-driven input shaft is to be coupled to an output shaft through either of two pinions forming part of a reversing gear. The coupling includes two jointly displaceable pistons moving in respective cylinders which are alternately connectable, through a selector valve, with the high-pressure side and the low-pressure side of a fluid source in order to establish either one or the other operating position.
In the hydraulic actuation of automotive clutches and brakes, which have become collectively known in the art as drive-establishing means, it is desirable to admit pressure fluid (referred to hereinafter as oil) to the working cylinder thereof at a controlled rate designed to soften the shock of the cut-in and to reduce the wear of the frictionally interengaging blades of the clutch or brake assembly. The gradual increase of the supply pressure, reaching its normal operating level a certain time after the shifting of the selector valve, is controlled by a timer, e.g. as disclosed in commonly owned application Ser. No. 177,320 filed Sept. 2, 1971 by Klaus Podssuweit and Winfried Felder, now U.S. Pat. No. 3,814,224. However, both the initial pressure and the rate of pressure buildup may vary as a result of fluctuations of such parameters as supply pressure and operating temperature.